This book is about electrical communications networks, including both analog, digital, and hybrid networks. We will look at both broadcast and bi-directional data networks. This book will focus attention on existing technology, and will not be concerned particularly with too much mathematical theory.

This book is an example-driven book. We will use examples of real world communication technologies and communication networks to teach and demonstrate some of the principles behind communication theory. We will discuss examples of communication networks, and introduce the various mathematical principles that those networks rely on.

A network can be of practically any size. The only practical limitations are those imposed by the protocols which it implements. A small home based network may be comprised of simply 2 computers which share a common connection to the internet. A larger example would be a corporate network where every employee in each department of the corporation has their own workstation, which they use to access not only the internet, but also the servers deployed throughout the network, and the workstations of other employees. Furthermore, a network device is not constrained to just a PC workstation. There are many devices which may be connected to a network, including routers, switches, bridges, access points, firewalls, etc.

The fundamental requirement for two devices is to use the same protocol for exchanging information. This is no different than human communication, for instance. In this analogy, the language used can be considered the protocol. If I want to initiate communication with someone else, the fundamental requirement is that we both know how to speak the same language, regardless of what that language is. In the same way, in order for two devices to exchange information, they must be aware of a common set of rules or specifications to communicate with each other. The primary purpose of a network is to exchange data. The devices connected to a network may have one (or more) of several roles in accomplishing that purpose. The most common would be your PC workstation or a server, where data is originated and stored. Other devices, such as a router, help in getting that data from one point on a network to another. In order to satisfy the requirements of a network, the original developers recognized the need for open standards so that any entity can contribute to, and use network technology. The International Organization for Standards (ISO) developed the Open Systems Interconnection (OSI) reference 7-layer model, which defines the standards for how networks operate.